This invention relates to a method for making oligomeric carbonate bischloroformate compositions. More particularly, it relates to a method for making oligomeric carbonate bischloroformate compositions wherein the use of phosgene is minimized.
Bischloroformate oligomer compositions and methods for their preparation and conversion to linear polycarbonates are known in the art. Reference is made, for example, to U.S. Pat. Nos. 3,646,102, 4,089,888, 4,122,112 and 4,737,573.
A principal advantage in the preparation of linear polycarbonates from bischloroformate oligomer compositions is the relative purity of the products. This is particularly true when the molecular weight of the polycarbonate is regulated by the use of an endcapping agent such as phenol, t-butylphenol or p-cumylphenol. The use of such endcapping agents in reaction mixtures using phosgene causes the formation of diaryl carbonates such as diphenyl carbonate as by-products.
It has been found that the presence of such diaryl carbonates may cause difficulties in molding operations. These may include problems in removing molded polycarbonate articles from the mold, in producing parts using rapid cycle times, and in producing parts without physically or optically flawed surfaces. Such problems can be particularly vexatious when regularity of shape of such molded articles is a prime concern, such as in the molding of optical disks. By the use of bischloroformate oligomers, formation of diaryl carbonates and the attendant problems are avoided.
According to the aforementioned U.S. Pat. No. 4,737,573, aromatic bischloroformate compositions are prepared by reacting phosgene with a dihydroxyaromatic compound in the presence of aqueous base and a substantially inert, substantially water-insoluble organic liquid. The reaction occurs under back-mixing conditions, i.e., in a tank reactor, and at controlled pH, the aqueous base being added at a rate to maintain the aqueous phase of the reaction mixture at a pH in the range of 8-11. Phosgenation times range from 10-30 minutes. Linear polycarbonates can then be prepared by interfacially reacting the bischloroformate composition with an interfacial polycarbonate formation catalyst, e.g., a trialkylamine, and an endcapping agent, e.g., phenol.
It would be desirable to improve this prior art method in two respects. First, the molar ratio of phosgene to dihydroxyaromatic compounds used therein is higher than is desired.
In the method of the prior art, the conditions under which phosgenation is conducted, i.e., pH levels of 8-11 and phosgenation times of 10-30 minutes, normally result in the formation of oligomers of about dimer or trimer size. Formation of these oligomers requires at least 3/2 to 4/3 moles phosgene, respectively, per mole of dihydroxyaromatic compound. Because phosgene is a dangerous material and its use imposes severe environmental and safety constraints on manufacturing sites, it is desirable to minimize the amount of phosgene used per unit of dihydroxyaromatic compound.
The following equation represents the stoichiometry of bischloroformate preparation from phosgene and a dihydroxyaromatic compound: ##STR1## wherein R is as defined hereinafter and n is the average degree of polymerization of the bischloroformate product.
As can be seen from the equation, the average degree of polymerization of the oligomers is related inversely to the molar ratio of phosgene to dihydroxyaromatic compound. Thus, it is desirable to prepare longer oligomers during the phosgenation step so as to minimize this ratio.
The second aspect for which improvement is desired in the prior art method described above is the necessity therein to completely remove the trialkylamine catalyst from the aqueous and organic liquids used in the polymerization step, prior to the recycling of these liquids to the phosgenation reactor. The complete removal of the trialkylamine from these liquids requires additional processing equipment and energy. It is desirable to conduct phosgenation effectively in the presence of aqueous and organic liquids containing a trialkylamine.